US3001582A

US3001582A - Molding form

Info

Publication number: US3001582A
Application number: US713762A
Authority: US
Inventors: Harold V Kindseth; John A Johnson; Thomas B Ludlow
Original assignee: Bemis Brothers Bag Co
Current assignee: Bemis Brothers Bag Co
Priority date: 1958-02-06
Filing date: 1958-02-06
Publication date: 1961-09-26
Anticipated expiration: 1978-09-26

Description

Sept. 26, 196.1 H. v. KINDSETH ETAL 3,001,582

MOLDING FORM Filed Feb. 6, 1958 3 Sheets-Sheet 1 IN VENTORS flARom M h INDSETH JbH/v A. JbHNSON Tfiolwlsfl. Luaww WMu LUM W ATToRgEv q' p 1961 H. v. KINDSETH ETA]. 3,001,582

MOLDING FORM Filed Feb. 6, 1958 3 Sheets-Sheet 2 iij gggg 26 27 /55 23 FIG. 6

INVENTOR3 HAROLD l KINDSETH Jb/IN A JbHNSO/V THOMAS B. L UDLOW 2 QIIW,M1

ATTORNE Y5 Sept. 26, 1961 H. v. KINDSETH ETAL 3,001,582

MOLDING FORM Filed Feb. 6, 1958 5 Sheets-Sheet 3 INVENTORS hARoLo K hINDS ETH JOHN A io/wvso/v 77/0M/46 B. Luoww United States PatentO Harold V.

and Thomas B. Bemis Bro. Bag Company,

poratiou of Missouri Filed Feb. 6, 1958, Ser. No. 713,762

9 Claims. (Cl. 162-411) This invention relates to molding forms. More particularly it relates to shaped perforated molds of the type in which a pressure differential across the surface of said mold causes the deposition of fibrous material on the surface thereof.

Molding forms made in accordance with this invention are suitable for compacting fibrous material of all types, such as felt or fibrous paper pulp or glass fibers or the like. The invention is especially suited for the molding of fibrous pulp suspended in a slurry and for the production of fiberglass preforms.

1 A-method widely used in construction molds for shaping Wet pulp consists of wrapping a metal screen over the surface of a metal form which has the shape of'the article to be molded, the metal form being provided with holes in its surface which communicate with a cavity in the back of the mold. 'Ihe cavity in turn is connected to a source of vacuum. When the mold is in use, the vacuum draws fibers in the region of the mold against the screen. j

. Themanufacture of prior molds is a time-consuming and painstaking operation requiring the drilling and countersinking of a large number of holes in the metal backing, which is usually a precision casting. These forms are often fabricated in sections which must be fastened together by bolts. The screen itself must be wrapped around the various portions of the backing and clamped imposition. The process is limited by the ex ant to which the screen can be shaped or bent around the form. Bending the screen around some intricate surfaces is difiicult, if not impossible. Furthermore, these metal screens deteriorate with use in a relatively short time and frequently rescreening of the molds is required. Parting lines between mold sections often occur in a direction crosswise to the direciton in which molded fibrous articles are to be removed. This causes the articles to stick on the mold, sometimes resulting in torn and useless items. More serious yet, if molded articles release poorly, stresses are set up in the items which, although not apparent when first removed from the mold, show up in the form of exaggerated warpage in the subsequent drying operation. This results in total loss of the material in the spoiled item, since the article, once dried, cannot economically be repulped.

Because the prior art backing members are relatively expensive and difficult to construct they are oftentimes stored as a matter of economic necessity for long periods of time. The backing members of this invention are relatively cheap, easily constructed and maybe considered disposable. At a later time if it is desired to reuse the particular screen, it is a simple one-step operation to reform the backing material. Sets of the perforated molding sheets of this invention may be nested together into a relatively small compact light bundle, thus greatly simplifying the storage problem.

simplified mold construction,

an m

A mold can be constructed by bonding together smal spherical particles like sand grains, glass beads, resinous beads or metal shot in such a manner that a relative por-i ous composition is formed. This improved mold greatly but, because it is possible for the fibrous material to plug the interstices or holes of these molds, the proper size and shape of the beads and the amount of bonding material used is critical. Changes. in the composition of the slurry may be suificient to cause the form to eventually become plugged with fibrous material.

These difiiculties in the prior art are overcome by the molding form of this invention comprising a perforated thermoplastic sheet formed to the desired shape of the article to be molded and optionally supported by a relatively strong rigid backing member.

It is a primary object of this invention to provide a mold for compacting fibrous material which requires no casting, drilling, threading and fitting of metal parts iri its fabrication.

it is another object ofthis invention to provide a mold for compacting fibrous material from a slurry to a pre? determined shape which is inherently resistant to acids, alkalies and other corrosive agents likely to be found in said slurry.

Itis a further object of this invention to provide a mold for compacting fibrous material to a predetermined shape including perforated thermoplastic sheet which may be given the desired shape by means of forming while in a heat softened condition.

Qther objects of the invention will as the description proceeds.

To the accomplishment of the foregoing and related ends, this invention then comprises the features heroine after fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various Waysin which the principles of the invention may be employed. I

The invention is illustrated by the drawings in which the same numerals refer to corresponding parts and in which:

FIGURE 1 is an elevation, partly in section, of one embodiment of the mold of this invention;

FEGURE 2 is an enlarged fragmentary section through the embodiment of the mold illustrated in FIGURE 1;

FIGURE 3 is a side elevation, partly in section, of another embodiment of this invention;

FIGURE 4 is an enlarged fragmentary section showing a preferred form of construction of the molds of FIG-j URES 3 and 6;

FIGURE 5 is a side elevation, partly in section, of still another embodiment of this invention;

FIGURE 6 is a side elevation, partly in section, ofstill further embodiment of the invention;

FIGURE 7 is a vertical section illustrating one method of forming the mold of this invention shown before form ing; and

FIGURE 8 is a section similar to FIGURE 7 after the sheet has been formed.

Referring to the drawings, FIGURE 1 illustrates a molding form according to this invention wherein the molding surface comprises a formed finely perforated thermoplastic sheet 10 which is strengthened by a backing become apparent i ing sheet 20 is provided with 3 member 11. large number of bodies 12 of granular material of relatively uniform size bonded together by a coating of resinous material 13 applied substantially uniformly over the granules. The backing member 11 is enclosed within a frame member or ring 14. The thermoplastic sheet is provided with a flange 15 by which it too is secured to the frame member 14 which in turn is secured to a support 16, as by bolts 17. The support 16 has in it a duct 18 to which a vacuum is applied. The entire moldingform with its supporting means is movably mounted so that it may be immersed in a pulp slurry. The vacuum provides a pressure differential across the surface of the mold through perforations 13: which causes the fibrous material of the'slurry to accumulate upon the surface of the sheetlll. Sheet 10, of course, is formed in a shape corresponding to the desired shape of the pulp molded product. Liquid from the slurry flows readily through the interstices between the granules of the backing.

As the fibrous material accumulates on the surface of the screen, the water passes through the perforations in the sheet 10 and porous backing member 11 and is exhausted through the duct 18. After the pulp article is molded, the molding form is removed from the slurry' and the vacuum is continued in order to remove excess water and to compact the fibers into a relatively dense self-suppoitingstructure. Positive air pressure suflicient to lift the fibrous mass from the surface of the screen inay then be applied through the duct 18 and the molded article is removed.

FIGURE 3 illustrates another embodiment of the inyention in which the perforatedthermoplastic sheet 10 is supported by means of a somewhat thicker and stronger perforated backing sheet 29 conforming in shape to molding sheet 19 and having relatively large holes 21 therein. The backing sheet 20 may be made of formable thermoplastic resinous material and may be formed by the same methods as the molding sheet 1-3, as hereinafter described.

The backing sheet 24 may be conveniently formed simultaneously with the sheet 10 but when this is done a sepathe like should be provided to insure that two sheets does not occur. The holes 21 maybe of any convenient size. One quarter inch holes onhalf inch centers were found to be satisfactory. The backing sheet 2% may be on the order of about one eighth of an inch thick. The thickness, of course, need only be. enough to produce the requisite strength. The backflange 22 by which it is afiixed to the frame 14 and support 16..

In FIGURE 4 there is illustrated a preferred optional form of construction of the backing sheetfor a molding form sheet. of the type illustrated in FIGURE 3. According to this preferred form of construction, the thicker perforated backing sheet 29A underlying the perforated forming sheet-1t) is provided on its outer surface rator sheet or fusing of the adjacent to the inner surface of the sheet 10 with a plurality of projections 23 or similar irregularitiesso that the forming or molding sheet 1% is held slightly spaced away from me regions in the backing sheet. between the perforations 21A. .In this manner, somewhat better and more uniform distribution of suction over the entire surface of'the sheet 10 is provided and as a result, there is a. lesser tendency for the sheet 10 to become clogged with pulp fibers. Substantially thesanre result may be achieved by roughening the surface of the backing, or scoring, knurling or the like.- 7

FIGURE 5 illustrates still another embodiment of the invention. In this embodiment the molding sheet 10A is thick enough so that no backing member is necessary to provide the requisite strength. Byway of example, the sheet 10A may be on the order of one eighth tothree sixteenths inch thick while the holes 19A may be on the order of 0.02 inch in diameter with about 700 holes per square inch. In this embodiment the sheet ltlA may be thinner for molding relatively small articles and thickor for molding relatively large articles. The sheet 10A The backing member is composed of a 7 backed by a somewhat may likewise be formed by any of the methods outlined hereinafter. It is provided with a flange 15A by which it is aifixed to frame 14 and support 16.

In FIGURE 6 there is shown a still further modified form, of molding form according to this invention. In this modification of the invention, the molding form is a cavity mold in contrast to the plug molds previously described. Although the molding surface in this form of the invention is constructedjn accordance with the embodiment illustrated in FIGURE 3, the forms of the embodiments of Fl-GURES 1 and 4 may likewise obviously be employed instead. The thermoplastic perforated sheet 10B which forms the mold ng surface is provided with a plurality of small perforations 19B. The sheet 10B is thicker perforated thermoplastic sheet 203 having somewhat larger and more widely spaced perforations 21B. The sheets are provided with supporting flanges 15B and 2213, respectively.

The cavity mold is enclosed in a trim ring 23 having a central opening corresponding generally to that of the cavity mold and having a surrounding upturned outer flange 24. The flanges of the perforated mold form sheets telescope into thecavity created by the flange 24 of the trim ring. The support or vacuum box 25 fits over the cavity mold and is provided with an outwardly extending flange 26 which telescopes into the trim ring on top of the flanges 22B and 15B of the perforated sheets. The mold may be secured together bymeans of bolts passing through the trim ring 23 and the flanges of the perforated sheets and vacuum box, or, alternatively, they may be adhesively secured together, or both belts andadhesive may be used. In either event, a head of adhesive 27 is desirably placed in the crack around the edges of the vacuum box to provide a seal against passage of water. The vacuum box 25 is provided with a duct 28 by which it may be connected to a suitable vacuum source.

Both vacuum box 25 and trim ring 23 are made by vacuum forming over a simple wooden pattern in much the same manner as the perforated sheets. The mold form constructed of resinous material offers advantages notonly in its manufacture but savings in labor result from its use. It. is more water resistant than molds formed in part from metal or wood with the results that no surface treatment is necessary. The vacuum box 25 may be readily formed to any desired shape and it may bedished out on its upper surface so that water may be more readily drained from it While the mold is iri an inverted position.

Referring now to FiGURES 7 and 8, there is her illustrated one method for forming the perforated sheets; 10, lilA and 10B, backing sheets 29, 20A and 20B, vaen um. box 25 and trim ring 23 to the desired shapes. According, to this method a thermoplastic sheet S, such as cellulose acetate, cellulose acetate butyrate, rigid vinyl, modified polystyrene, polyethylene-An general, any thermoplastic material whichis commonly used in the sheet plastic forming artsis perforatedv in a suitable mesh. size. To this thermoplastic sheet is applied an imperforate film 31 of similar material'or of material from the group outlined above, optionally covered with a thin layer of an adhesive film 32. This impervious structure can then be vacuum formed around or into a pattern 33 of the desired shape which is prepared beforehand and whichmay be made from any of the materials commonly used inthe vacuum forming art. 7

The male pattern 33 made, for example, of wood rests on a hollow box 34, inverted so as to contain vacuum chamber 35. In this instance the surface of the male pattern 38 conforms to the desired inner surface of the molded pulp article, due allowance being made for the thickness of the plastic forming sheet. Holes 36 on the order, for example, of about 0.030" diameter, are drilled as shown through the pattern base 37 around the bottom of the pattern 33 and spaced, for example, about /2" apart. The

s nuses height of the box 34 is such that in the draping operation; the draping clamp preferably brings the plastic sheet about below the top surface of the pattern base 37.

A heater 38, which can be any of the various types conventionally used in the vacuum forming art, such as radiant panel or infra red heaters is mounted on rollers which ride in a track so that the heater may be rolled over a draping frame 39 including a series of clamps 40 for holding the sheet S and film 31. After the heater 38 has raised the laminate to forming temperature, the frame 39 is lowered to the position indicated in FIGURE 8 and a vacuum is created in chamber 35. The suction draws the softened films against the pattern surface. The laminate then conforms to the surface of the male pattern 33 and is allowed to cool. The sealing

films

31 and 32 may now be removed and the molding sheet is prepared for insertion in a mold.

Where a mold form is being constructed according to the embodiment of FIGURES 3 or 6, the molding surface and'backing may be formed together and then the thermoplastic sheet S is a composite of a relatively thinner finely perforated thermoplastic sheet, such as sheet 10 or 10B, in combination with a relatively thicker and more coarsely perforated backing sheet of thermoplastic material, such as sheet or 20B, with a thin separator film in between to prevent fusion of the perforated sheets. The thinner finely perforated sheet must always be placed between the heavy perforated sheet and the pattern. The fine sheet will draw down into the large holes of the heavy sheet if placed adjacent the side of the heavy sheet opposite the pattern. Alternatively, the sheets may be formed separately. When the piece being vacuum formed is, for example, to be used as a vacuum box, then the'thermoplastic film S is not perforated and use of the imperforate film 31 is no longer necessary.

If it is desired to maintain exact dimensions on the inside of the thermoplastic sheet, one would drape form over a male plug. Conversely, to maintain dimensions on the outside of the thermoplastic sheet one would vacuum form into a cavity. If dimensions of the finished article are not critical enough to necessitate allowing for the thickness of the plastic, it may be desirable to drape form even though the desired molding surface is to be the outside of the perforated sheet. Thus, for example, if one desired to attain maximum strength in the bottom of a box-shaped sheet, one would drape form because the smallest amount of stretch would occur in that section. Conversely, instances occur in which forming into a cavity would be the preferred method, regardless of which side of the sheet is to be the molding surface. These considerations and various other techniques will, of course, be immediately apparent to those skilled in the art of vacuum forming.

Although elementary vacuum forming techniques lend themselves well to our invention, we do not intend that the invention be limited to such. Any method of forming a perforated plastic sheet would enable one to produce the desired molding surface. Thus, one may use matched dies to mechanically form a preheated plastic sheet. Simple geometrical shapes may be produced by line bending techniques. To produce a molding surface for molded pulp packaging forms the thermoplastic sheet may be drape formed around the article to be packaged; i.e., the article to be packaged would serve as the vacuum forming pattern. If this method is adopted the pattern 33 may be dispensed with.

One important feature of our invention is that not only can a perforated molding surface be readily formed, but it may be produced as a separate, self-supporting unitary sheet. This allows replacing the expensive, machined castings used in the prior art as supporting mediums, with the easily constructed backing of our invention. Other advantages accruing from the use of the integral sheets of this invention are pointed out hereinafter.

It can thus be seen that this invention provides an exeel-lent method for forming flat perforated resinous thef-" moplastic sheet material to any desired shape. Besides being used in molds, the formed perforated resinous sheets are useful in themselves for other purposes. They; may be used, for example, as receptacles such as berry boxes or as sieves or strainers or even as components in furniture such as lawn chairs or the like. Other uses for' this formed perforated thermoplastic material will be apparent.

The thickness .of the plastic sheets 10, 10A and 10B" is determined on one hand by the degree to which the thermoplastic sheet must be stretched in order to form the desired molding surface. That is to say, if it is to be stretched to a great extent it must be relatively thick in order that the portions of maximum stretchwill be strong enough to withstand conditions encountered during use." 0n the other hand maximum thickness is determined only to the extent that the material can be perforated in the desired mesh. We have found, for example, that cellulose acetate butyrate 0.040" thick, can be cleanly and inexpensively perforated in 0.020" diameter holes, upto about 1000 holes per square inch and is made commercially available by companies who specialize in perforating sheet material. Machines designed specifically for perforating soft plastics, may be made to handle much thicker sheets, such as the embodiment illustrated in FIGURE 5. v I

The hole size of the finished molding surface should range from about .015" to aboutLOSO" in diameter. In order to meet this condition one is limited as to how deep or extreme a structure can be vacuum formed without unduly distorting the perforations. To form molding surfaces in which a great deal of stretching must take place one may vacuum form the desired moldingsurface in sections and subsequentlycement or fasten these sec: tions together to form a continuous sheet by simple means well known in the plastics art. These joining lines can be arranged so that they run parallel to the direction-in which molded articles are to be removed, thereby allowing smooth, easy release of the molded article.

The supporting structure 11 of the embodiment illustrated in FIGURE 1 can be constructed from particles of any material which is inert to the environment in which the molding form is to be used, such as glassbeads, metal, shot, plastic beads and pellets, and gravel. These particles need not be perfectly regular nor uniform in shape. Nor is their size critical. They may be to /2" or greater. For pulp molding purposes, they should not be so small that they tend to entrap minute pulp fibers which pass through the perforated sheet. On the other hand, they should be small enough so that the per-, forated plastic sheet is supported at enough points to prevent the surface of the sheet from being distorted during the vacuum phase of the molding cycle. To illustrate the wide variations permissible in size and shape; gravel particles containing about 20% crushed material, screened to range in sizes from A3" to and cemented together to form a porous structure with walls about thick, produce a supporting member which does not tend to fill with pulp fibers and yet provides adequate support for a plastic molding surface.

The invention is further illustrated by the following example in which there is described the construction of a molding form suitable for use in producing molded pulp plantersbox shaped containers used in plant nurseries to hold growing plants and the dirt required for their growth. These containers have the smooth finish on the inside. Consequently they are produced with male molding forms. This example, then, will describe the production of a male form. Application of our invention to construction of female type molding forms will be readily apparent to those skilled in the art of pulp molding, and/or the art of vacuum forming.

A sheet of cellulose acetate butyrate, 0.040 thick was perforated with holes 0.02 inch in diameter with about 700'holes per square inch. The holes in the sheet were temporarily sealed off so that vacuum could be applied during the forming operation by laying an impermeable and imperforate sheet of polystyrene over the perforated sheet; Polystyrene sheet having a thickness of 0.020 inch was used. The laminate so produced was clamped into the draping frame. The heater was then. moved into position over the draping platform. When the laminate reached forming temperature, the draping frame was brought down, causing the softened laminate to drape or stretch around a suitably shaped wooden male pattern. Asthe draping frame reached the lowest position in its "travel, a vacuum was applied through a vacuum line into the vacuum chamber and through the vacuum holes in the mold to the hot stretched laminate. In this manner the sheet was drawn snugly around the male pattern and after setting was removed. In cases where the imperforate sheet which is used for sealing the perforated sheet during the forming operation is on the sideof the molding sheet which normally rests against the backing, the formed molding sheet itself may be used as a patternfor the'backing. The sealing sheet then acts as a release sheet. Otherwise, the formed sheet may be treated to readily release the backing or a separate pattern may be used.

In this instance, into the drape formed perforated molding sheet was packed a mixture consisting of pieces of gravel ranging from about is" to A in diameter uniforrnly admixed with 4% by weight of an adhesive which consisted of epoxy-polyamide in the ratio of 7 to 3. This adhesive set up spontaneously at room temperature to from a backing structure which is self-supporting. If a greater porosity in the backing member is desired, a solid wooden core, dimensioned to yield approximately 22" walls, may be sunk in the freshly mixed gravel-adhesive mixture. After about 6 hours, the aggregate was removed from the perforated molding sheet pattern and post-cured for about one hour at 300 F. The backing member and the perforated sheet was then recombined and mounted on a suitable vacuum box support as described above for use in a conventional pulp molding apparatus. We were surprised to find that in spite of the fact that some of the holes of the molding sheet are distorted or enlarged in the process of forming, the sheet still funct-ions well, is not subject to plugging and can easily be kept clean. Furthermore, the molds may be prepared in a; simple and straightforward fashion and do not require the care of a skilled craftsman. The vacuum forming rnachine required for this operation does not need to be nearly as complex as a commercial. vacuum forming machine. A simple combination of heat source, vacuum pump and an adjustable draping platform are sufiicient for the needs of this invention. Because the number of new molding sheets required per day in the pulp molding industry is small in comparison to output of commercial vacuum forming machines, such elaborate vacuum forming machines are unnecessary and the molding sheets may be produced with a minimum of equipment and with the use of inexpensive manual controls.

Furthermore, since the thermoplastic perforated molding sheet does not have to be fixed to the backing, thin sheets tend to flex outward from the backing when positive airpressure is applied to release the molded article. This effectis particularly beneficial in female molding forms since the flexing action tends to help loosen. the molded article from the molding surface. This in turn promotes good release, thus reducing subsequent warpage.-

The mold herein. described was found to have a num-' berof advantages over prior art molds. Once the easily constructed wooden pattern is made, a large number of molds may be made, rapidly and economically. This invention permits the manufacture of molds whose shapes areso complex. as to be impossible to make by existing methods. The molds were found. to be unaffected by acids or other jcorrosive agents used inthe molding process. They were strong and, durable enough to last indefinitely, and, were not subject to becoming plugged with fibersa.

it is apparent thatfmany modifications and variations of this invention, as 'hereinbefore set forth, may be made without departing from the spirit and, scope thereof.

The specific embodiments described are, given by way of example only, and the'invention is limited only in terms of the appended claims.

We claim:

l. A mold for forming fibrous pulp articlm comprising a rigid perforated thermoplastic and thermoformable,

resinous sheet formed to the shape of an article to be molded, supporting means for said sheet and means cornmunicating through said supporting means for applying sucticn'tosaid mold.

2. A mold. according to claim 1 further characterized that the supporting means for said formed perforated thermoplastic resinous sheet comprises a formed imperforate thermoplastic resinous sheet secured at its edges to' said perforated sheet and spaced from the perforated sheet away from said edges to define a chamber in direct communication with the openings in said perforated sheet, and at least one opening in said formed imperforate sheet for applying suction to said chamber.

3. A mold according to claim 2 further characterized in that the central portion of said formed imperfor'ato thermoplastic resinous sheet is deformed out of its normal plane, said sheet includes an outwardly extendingiiange projecting from the periphery of the sheet, and said support means includes a ring corresponding gener ally in shape tosaid flange and attached thereto to secure said perforated sheet to said support.

4. A mold according to claim 3 further characterized in that said ring is provided with an upwardly extendingflange at its outer periphery and the flange on said formed imperforate sheet fits telescopically therein.

5. A mold according to claiml further characterized in the provision of a porous; backing manner in contact with the inner surface of said perforated sheet, said back-- ing member being substantially rigid and having suflicient strength to prevent deformation of the perforated sheet in the course of normal use of the mold in pulp molding.

6. A rnoldof the type in which the article to be molded is. deposited as a fibrous mass by means of a pressure differential across the surface of said mold comprising a rigid perforated thermoplastic resinous sheet formed to the shape of the article 'to be molded, said sheet having sufiicient strength to withstand s'aiddiiierential pressure without deformation, a support means for said mold and means communicating through said support means for applying suction to said mold.

7. A mold of the type in which the article to be molded is deposited as a fibrousmass by means of a pressure difierential across the surface of said mold comprising a rigid perforated unitary thermoplastic resinous sheet formed to the shape of the article to be molded, a porous back ng member in contact with the inward surface of said sheet, said backing member having sufficient strength to prevent deformation of said perforated thermoplastic sheet by said pressure differential, a support means for saidsheet and said backing member and means communicating through said support meansfor applying suction to said mold.

8. Arnold according to claim 7 wherein said porous backing member comprises a relatively thick thermoplastic resinous sheet, the pores in said backing member comprising spaced openings of about Mi inch in diameter and about Z2 inch on centers.

9. A mold according to claim 7 wherein said backing member comprises a rigid porous shell of particulate aggregate bonded together by adhesive, one surfaceof said backing member corresponding to the shape of the adjacent surface of said thermoplastic sheet.

References Cited in the file of this patent UNITED STATES PATENTS 5 342,176 Carmichael May 18, 1886 2,187,918 Sloan Jan. 23, 1940 2,320,789 Montague et a1 June 1, 1943 2,424,189 Randall July 15, 1947 10 10 Shepard Feb. 12, 19 52 Michiels Mar. 2, 1954 Lambert Sept. 27, 1955 Kyle Nov. 15, 1955 Chellis June 25, 1957 Van Hartesveldt July 8, 1958 FOREIGN PATENTS Australia Aug. 12, 1943 Great Britain Aug. 17, 1949